1. Field of the Invention
The present invention relates to electrographic imaging methods and apparatus of the type utilizing an image member having an electrically conductive, reference-potential layer sandwiched between electrically insulative layers, and more particularly, to methods and apparatus for producing electrical contact with such sandwiched, reference-potential layer.
2. Description of the Invention Relative to Prior Art
In electrographic imaging, image quality is dependent upon establishing a reference potential, e.g. ground or another predetermined reference level, for the image medium. For this purpose, an electrically conductive layer is commonly incorporated in the imaging medium between an insulative film support and a photoconductive insulator layer. Various means have been used to establish and maintain the necessary reference potential on such an insulatively sandwiched conductive layer. U.S. Pat. No. 4,344,698 discloses a useful knife-edge grounding apparatus for this purpose. However, cutting through the insulating material damages the web and may lead to contamination of the image area by the cutting debris. Also, the knife sharpness must be maintained.
A reference potential may be provided without mechanical contact with the web by a corona discharge as described in U.S. Pat. Nos. 2,955,938; 3,620,614 and 3,738,855. The high voltage apparatus required to provide the desired potential difference by the corona discharge method is expensive and requires periodic readjustment and maintenance.
Various mechanical contact means have also been disclosed for use with specially configured webs. For example, one or more insulative layers in these webs may be bridged by added conductive material, as disclosed in U.S. Pat. Nos. 3,639,121 and 3,743,410. Alternatively one or more insulative layers may have discontinuities that provide direct access to the conductive layer, as disclosed in U.S. Pat. Nos. 3,533,692 and 3,910,475. However, the complex structure of such specially configured webs increases the cost of coating and finishing operations required to manufacture the web. From the fabrication viewpoint, it is advantageous to coat a wide web uniformly from edge to edge without striping, edging or pattern screening steps and subsequently slit it to narrower widths. During such slitting operations portions of the photoconductor or support layer frequently are displaced over the edge of the conductive layer and further isolate it from electrical contact.